(a) If any rate of tax imposed by this article changes to
become effective after the thirty-first day of December, of a
calendar year, and if the taxable year includes the effective date
of the change of rate (unless that date is the first day of the
taxable year) then: (1) Tentative taxes shall be computed by
applying the rate for the period before the effective date of the
change of rate, and the rate for the period on and after such date,
to the taxable income for the entire taxable year; and (2) the tax
for such taxable year shall be the sum of that proportion of each
tentative tax which the number of months in each period bears to
the number of months in the entire taxable year.

(b) For purposes of subsection (a):

(1) If the rate changes for taxable years "beginning after" or
"ending after" a certain date, the following day shall be
considered the effective date of the change; and

(2) If a rate changes for taxable years "beginning on or
after" a certain date, that date shall be considered the effective
date of the change of rate.